In the specific case of blast furnaces, the internal refractory lining is conventionally made up of an assemblage of a plurality of refractory elements, in the form of blocks or bricks, of identical or different sizes and shapes depending on the location of each block or brick. The material of which these refractory elements are composed also depends on where they are located in the stack. Accordingly, the hearth floor, or bottom, is commonly fitted with a plurality of superposed layers of elements, for example two or three layers of carbon blocks surmounted by two layers of ceramic elements, the upper layer forming the hearth floor which is in contact with the molten iron. The refractory lining of the side walls of the hearth is itself also usually formed from a superposed arrangement of a plurality of annular refractory layers. In the lower part, each layer may comprise an outer annular part, located closest to the metal wall of the stack, and an inner annular part, which forms the side wall of the hearth in contact with the molten iron, or closest thereto. Each annular part is formed of a plurality of circumferentially juxtaposed blocks and the material of the various blocks is adjusted to its location in the various layers and rings. Furthermore, elements of a specific shape and material are used at the level of the tapholes and also, higher up, at the level of the tuyeres.
The ramming masses are used in constructing the internal refractory lining of the stack for filling the gaps between the refractory elements, which are blocks or bricks, typically made of carbon or ceramic, and the walls or other metal elements forming the blast furnace stack, as well as for filling the gaps between adjacent refractory elements or between different layers or courses of such elements, and in particular between certain refractory elements of the bottom and certain other elements of the side walls of the hearth.
Depending on their location, there will thus be “cold” joints between the blocks of the outer rings and the cooling panels, and hot joints between the blocks of an inner ring and those of an outer ring.
The distance between an inner ring and an outer ring may be substantially constant, for example of the order of 50 mm, with a precision of a few mm as a result of the good dimensional tolerance of the blocks.
In contrast, due to the geometric irregularities of the stack, in particular the imperfect circularity thereof resulting from the much larger tolerances of the cooling panels of the stack wall, the distance between the blocks of an outer ring and the cooled stack walls may vary substantially around the circumference, the thickness of the joint to be produced typically being 80 mm, ±20 mm.
Moreover, account must be taken of the elevated temperature differential, which prevails in service, between the internal wall of the hearth and the external face of the lining, located close to the metal wall of the stack, and of the variation in this temperature differential over the various operating phases, in particular during start-up. This is because when the lining blocks are initially laid they are all at low temperature, whether they are located towards the inside of the hearth or towards the stack wall, whereas when the blast furnace is in operation the outermost blocks remain relatively cold, while those in contact with the molten iron are heated to very high temperatures. Some variation in the gaps between the blocks must therefore be allowed, in particular in the radial direction, between the blocks of two concentric rings, in order to offset the significant differentials in expansion when, in particular during commissioning, the inner layer of the lining heats up much faster than the outer layer. It is therefore highly advantageous for the joint between two rings to be compressible. By way of example, the compressibility of the joints between these two layers may amount to 15 to 20%.
At the level of the joints between the inner rings of the wall and the upper layers of the bottom, the gap may also be quite irregularly shaped, in particular due to the shape of the bottom blocks or bricks which are arranged in accordance with a plan, but possibly being assembled in this plan in parallel lines, whereas the blocks of the ring are arranged in a circle. Apart from the significant dimensional variations in these joints, which may also be known as hot joints, the joints are furthermore created between blocks which may be made of different materials, for example carbon for the ring blocks and ceramic for the bottom, which results in different behaviour in the event of any temperature fluctuations, in particular during the start-up phase of the blast furnace.
It is therefore necessary to use ramming mass to compensate any irregularities which may leave gaps between adjacent blocks, the mass therefore acting to fill the irregular and/or variable gaps between the blocks. The ramming mass also has the function of ensuring satisfactory thermal transfer from the hot internal face of the lining towards its cooled external face, and of at least not disrupting the thermal transfer which takes place through the refractory elements. The ramming mass also, importantly, has the function of providing a compressible body, in the manner of a plastically compressible joint capable of absorbing and reducing the thermomechanical stresses which may develop between refractory elements, in particular due to temperature fluctuations, especially between the bricks of the bottom and the inner layer of blocks of the side lining, as well as between said inner layer and the outer layer of this lining.
Known ramming masses are commonly composed of a mixture of a granular phase composed of calcined anthracite or electrographite, or a mixture of these, with the possible addition of SiC, and a binder generally of the tar or resin type, with the aim of providing the above-stated compressibility.
Various ramming mass compositions are already known for example from JP 2002121080, CN 1544389, CN1690012 or CN101823891.
One general problem with ramming masses is that they are generally placed by manual compaction, possibly assisted by compacting machines, but they are always less homogeneous and less compact than the refractory blocks or bricks manufactured by sophisticated pressing or extrusion methods. Since the granular phase is furthermore essentially based on standard carbon or graphite, the erosion and/or corrosion resistance of the joints formed by these ramming masses when exposed to molten metal and aggressive agents such as alkalis, steam etc. is much lower than that of the prefabricated refractory elements.
Furthermore, the commonly used tar type binder may remain relatively flexible up to a temperature of the order of 500° C., but at higher temperatures, for example from approximately 800° C., it polymerises and forms a very strong bond between the granules of the ramming mass, so destroying its relative flexibility. Accordingly, prior art ramming masses comprising a binder of this type may be used at the level of “cold” joints, since in this case the relatively low temperature makes it possible to preserve the required compressibility of the joint, particularly between the outer rings, or outer refractory layers, and the cooled metal walls of the stack. In contrast, at the level of the above-mentioned hot joints, hardening of the joint leads, above a temperature of the order of 500° C., to a loss of compressibility. This hardening of the joint may also lead, in particular at the level of the joints between the bottom and side wall, to an increased risk of infiltration due to cracking of the joints.